mjhjmtu

Consider the following script:

#!/usr/bin/env bash

set -ex

wget -N http://archive.ubuntu.com/ubuntu/pool/universe/o/ocaml-re/ocaml-re_1.7.3-2.dsc
wget -N http://archive.ubuntu.com/ubuntu/pool/universe/o/ocaml-re/ocaml-re_1.7.3.orig.tar.gz
wget -N http://archive.ubuntu.com/ubuntu/pool/universe/o/ocaml-re/ocaml-re_1.7.3-2.debian.tar.xz
dpkg-source -x ocaml-re_1.7.3-2.dsc
cd ocaml-re-1.7.3
dpkg-buildpackage -rfakeroot -d -us -uc -S || exit $?
debuild -S || exit $?

When I run it, I see

+ wget -N http://archive.ubuntu.com/ubuntu/pool/universe/o/ocaml-re/ocaml-re_1.7.3-2.dsc
--2018-10-28 20:29:03-- http://archive.ubuntu.com/ubuntu/pool/universe/o/ocaml-re/ocaml-re_1.7.3-2.dsc
Resolving archive.ubuntu.com (archive.ubuntu.com)... 91.189.88.162, 91.189.88.149, 91.189.91.23, ...
Connecting to archive.ubuntu.com (archive.ubuntu.com)|91.189.88.162|:80... connected.
HTTP request sent, awaiting response... 304 Not Modified
File ‘ocaml-re_1.7.3-2.dsc’ not modified on server. Omitting download.

+ wget -N http://archive.ubuntu.com/ubuntu/pool/universe/o/ocaml-re/ocaml-re_1.7.3.orig.tar.gz
--2018-10-28 20:29:03-- http://archive.ubuntu.com/ubuntu/pool/universe/o/ocaml-re/ocaml-re_1.7.3.orig.tar.gz
Resolving archive.ubuntu.com (archive.ubuntu.com)... 91.189.88.152, 91.189.88.162, 91.189.88.149, ...
Connecting to archive.ubuntu.com (archive.ubuntu.com)|91.189.88.152|:80... connected.
HTTP request sent, awaiting response... 304 Not Modified
File ‘ocaml-re_1.7.3.orig.tar.gz’ not modified on server. Omitting download.

+ wget -N http://archive.ubuntu.com/ubuntu/pool/universe/o/ocaml-re/ocaml-re_1.7.3-2.debian.tar.xz
--2018-10-28 20:29:03-- http://archive.ubuntu.com/ubuntu/pool/universe/o/ocaml-re/ocaml-re_1.7.3-2.debian.tar.xz
Resolving archive.ubuntu.com (archive.ubuntu.com)... 91.189.88.152, 91.189.88.162, 91.189.88.149, ...
Connecting to archive.ubuntu.com (archive.ubuntu.com)|91.189.88.152|:80... connected.
HTTP request sent, awaiting response... 304 Not Modified
File ‘ocaml-re_1.7.3-2.debian.tar.xz’ not modified on server. Omitting download.

+ dpkg-source -x ocaml-re_1.7.3-2.dsc
dpkg-source: warning: extracting unsigned source package (ocaml-re_1.7.3-2.dsc)
dpkg-source: info: extracting ocaml-re in ocaml-re-1.7.3
dpkg-source: info: unpacking ocaml-re_1.7.3.orig.tar.gz
dpkg-source: info: unpacking ocaml-re_1.7.3-2.debian.tar.xz
+ cd ocaml-re-1.7.3
+ dpkg-buildpackage -rfakeroot -d -us -uc -S
dpkg-buildpackage: source package ocaml-re
dpkg-buildpackage: source version 1.7.3-2
dpkg-buildpackage: source distribution unstable
dpkg-buildpackage: source changed by Mehdi Dogguy <mehdi@debian.org>
 dpkg-source --before-build ocaml-re-1.7.3
 fakeroot debian/rules clean
dh clean --with ocaml
 dh_testdir
 dh_auto_clean
 make -j1 clean
make[1]: Entering directory '/home/jgross/Documents/repos/coq-debian-build-scripts/ocaml-stuff/debian-sources/foo/ocaml-re-1.7.3'
make[1]: Leaving directory '/home/jgross/Documents/repos/coq-debian-build-scripts/ocaml-stuff/debian-sources/foo/ocaml-re-1.7.3'
 dh_ocamlclean
 dh_clean
 dpkg-source -b ocaml-re-1.7.3
dpkg-source: info: using source format '3.0 (quilt)'
dpkg-source: info: building ocaml-re using existing ./ocaml-re_1.7.3.orig.tar.gz
dpkg-source: info: building ocaml-re in ocaml-re_1.7.3-2.debian.tar.xz
dpkg-source: info: building ocaml-re in ocaml-re_1.7.3-2.dsc
 dpkg-genchanges -S >../ocaml-re_1.7.3-2_source.changes
dpkg-genchanges: not including original source code in upload
 dpkg-source --after-build ocaml-re-1.7.3
dpkg-buildpackage: binary and diff upload (original source NOT included)
+ debuild -S
 dpkg-buildpackage -rfakeroot -d -us -uc -S
dpkg-buildpackage: source package ocaml-re
dpkg-buildpackage: source version 1.7.3-2
dpkg-buildpackage: source distribution unstable
dpkg-buildpackage: source changed by Mehdi Dogguy <mehdi@debian.org>
 dpkg-source --before-build ocaml-re-1.7.3
 fakeroot debian/rules clean
dh clean --with ocaml
 dh_testdir
 dh_auto_clean
 make -j1 clean
make[1]: Entering directory '/home/jgross/Documents/repos/coq-debian-build-scripts/ocaml-stuff/debian-sources/foo/ocaml-re-1.7.3'
make[1]: jbuilder: Command not found
Makefile:12: recipe for target 'clean' failed
make[1]: *** [clean] Error 127
make[1]: Leaving directory '/home/jgross/Documents/repos/coq-debian-build-scripts/ocaml-stuff/debian-sources/foo/ocaml-re-1.7.3'
dh_auto_clean: make -j1 clean returned exit code 2
debian/rules:13: recipe for target 'clean' failed
make: *** [clean] Error 2

I am baffled by the fact that, although I have jbuilder installed (via opam), debuild -S cannot find it, even though it seems to just be running dpkg-buildpackage -rfakeroot -d -us -uc -S which succeeds just fine. What's going on here?